Should I use U-net to label keys in a keyboard image?
If you just need to draw a rectangle around each key, this is an object detection or template matching problem, so you can use any of the available models for object detection (e.g. YOLO) or any technique for multi-template template matching (e.g. you can use sequential RANSAC or t-linkage). In the first case, you will need a labeled dataset, while, in the second case, you will need the original image and the templates (in your case, a template would be an image of a key).
So, no, this is not a segmentation problem (which would be the task of classifying each pixel in the objects of interest, and not just locating the objects).